In the fabrication process for a semiconductor device, numerous processing steps, i.e., as many as several hundred, must be performed on a semiconducting wafer to form the device. Among the numerous processes, a wafer is frequently positioned on a rotating wafer chuck, or a vacuum chuck, for carrying out the specific process. Among the numerous examples of such processes are the photoresist coating process, the photoresist developing process and an edge bead rinse process for removing excess coating.
In a typical photoresist liquid coating process, a small amount of a photoresist liquid is dispensed onto a wafer that rotates at high speed from a dispense nozzle at the center of the wafer.
Since only a very thin coating of the photoresist material is normally required, the amount of the liquid photoresist material dispensed is small. When a liquid dispense nozzle is not perfectly centered in relation to a rotating wafer, incomplete coverage or even voids on the wafer surface can occur. Such poor coverage of a photoresist coating on a wafer surface results in a high scrap rate of the devices formed on the wafer or even the scrap of the whole wafer.
After the photoresist coating process, a wafer is imaged in a stepper to reproduce circuits desired on the wafer. A liquid developer material is then dispensed onto the surface of the wafer in a technique similar to that used in dispensing the photoresist material. A perfectly centered dispensing nozzle for the developing liquid is crucial to the successful developing of the photoresist layer in order to reproduce the circuits.
After a photoresist material is coated, exposed and developed on the wafer surface, an edge bead rinse (EBR) process is frequently performed before the wafer can be further processed. This is because that, in most processing chambers, a clamp ring is used to hold the wafer down on a platform during a chemical or physical process. A peripheral edge on the top surface of the wafer that will be overlapped by the clamp ring must therefore be cleaned of the photoresist material so that the photoresist material does not crack under the clamp ring and possibly causing serious particulate contamination problems. The edge bead rinse process is an important step that must be carried out after a photoresist coating and developing process.
The wafer processing steps that involve the photoresist coating, developing and edge bead rinsing can be performed in a variety of commercially available process machines. One of such process machine is a WAFER-TRACK.RTM. machine 10, shown in FIG. 1, for loading/unloading wafers. A long belt-driven main arm 12 is used to load or unload wafers to and from various processing stations, i.e., a photoresist coating station, a photoresist developer coating station and an edge bead rinse station (not shown). The main arm 12 is driven by belt 14 to and from each of the stations. The main arm 12 is capable of moving in X, Y, Z and .theta. directions as shown in FIG. 1. An enlarged view of the main arm 12 is shown in FIG. 2. The main arm 12 is constructed by three blades, X1, X2 and X3 to perform the various functions of loading and unloading. In operation, main arm 12 moves by belt 14 to a specific station, one of the blades then swings 90.degree. and extends through a loading slot of the process station to load or unload a wafer. The blades are equipped with an inclined top surface such that a wafer positioned on top may slide off and be positioned on a vacuum chuck.
A typical vacuum chuck employed in a process station is shown in FIGS. 3 and 4. Vacuum chuck 20 consists of a circular-shaped disk 22 having a top surface 24 equipped with vacuum slots 26. The vacuum slots 26 are in fluid communication with a vacuum passage 28 provided in a hollow shaft 30 which is unitarily connected to the center of a bottom surface 18 of the circular-shaped disk 22. The shaft 30 further consists of a hollow end 32 for connecting to a motor shaft 36 for rotational motion.
As shown in FIG. 3, the vacuum chuck 20 may be removed by hand by pulling upward from or installed by pushing downward onto the motor shaft 36. The vacuum chuck 20 shown in FIG. 3 may be employed in any of the processing stations for photoresist coating, developer coating or edge bead rinsing. The vacuum chuck 20 is not equipped with any means for ensuring the proper centering of a wafer positioned on top of the chuck.
In a conventional wafer loading process, as shown in FIG. 4, main arm 12 delivers a wafer 40 into a process station through a loading slot 42. After the wafer 40 is loaded on top of a vacuum chuck (covered by the wafer and not shown in FIG. 4), the main arm 12 withdraws from the loading slot 42. The movement of the main arm 12 is controlled by a process controller through the input of digital signals by a machine operator. The only way for checking whether the wafer has been positioned, or centered, properly on the vacuum chuck is to rotate the wafer 40 by hand. This is shown in FIG. 4. If the wafer is not positioned concentric with the vacuum chuck as shown by an off-centered rotation of the wafer, the machine operator inputs a new set of digital signals into the process controller based solely on his experience. There is no visual reference available to the machine operator for making an objective determination. The process is normally repeated several times before the movement of the main arm in positioning the wafer is properly centered. It is a trial and error process which requires a high skill level on the machine operator. The procedure is therefore both time consuming and highly operator dependent.
In a conventional edge bead rinse process which is conducted in a separate processing station, as shown in FIG. 5, a nozzle assembly 46 controlled by a digital step motor 48 is used to wash the edge bead on a wafer surface. Each bit or pulse by the digital motor 48 causes a displacement of the nozzle head 50 by 0.31 mm. For an 8 inch wafer, a width of 1.62 mm is normally required for edge bead rinse on the outer edge of the wafer.
When a photoresist coated wafer is placed on a vacuum chuck for the edge bead rinse process, the centering of the wafer in relation to the vacuum chuck is very important. Improperly centered wafer results in an unevenly washed wafer edge with one side of the wafer having excessive photoresist coating on top which may result in serious particulate contamination problems in future processing steps. To prevent the uneven wash of a wafer in an edge bead rinse process, a machine operator must provide data inputs to the main arm for positioning the wafer before a correct position can be found. The trial and error process is both labor and time consuming. Moreover, when the centering process is not correctly performed, the yield of the edge bead rinse process will suffer.
FIG. 6 shows a conventional process for dispensing a liquid material, i.e., a photoresist liquid or a developer liquid, onto a wafer surface. A dispensing nozzle 54 is used to dispense a small amount of liquid at the center 56 of wafer 40. The distance between the top surface 58 of wafer 40 and the tip 60 of the dispensing nozzle 54 is kept at a minimum, i.e., approximately 4 mm in order to minimize air bubble or turbulent flow. In a conventional process, after a main arm 12 delivers wafer 40 to the vacuum chuck 20, the machine operator must do trial runs on a few wafers before a correct positioning of the nozzle tip on the wafer can be achieved. This is done by observing the formation of any uneven flow of the liquid on the surface of the wafer, any void formation on the surface or uneven thickness of the photoresist layer formed on top of the wafer. The conventional process of centering a wafer on a vacuum chuck is therefore inadequate and costly for the proper centering of a wafer on a vacuum chuck.
It is therefore an object of the present invention to provide a method and apparatus for centering a wafer processing machine that utilizes a rotating wafer platform that does not have the drawbacks or shortcomings of the conventional methods or apparatus.
It is another object of the present invention to provide a wafer centering device for centering a wafer processing machine utilizing a rotating platform such that the practice of a trial and error method can be eliminated.
It is a further object of the present invention to provide a wafer centering device for centering a wafer processing machine that utilizes a rotating platform for use in conjunction with a wafer loading/unloading robot arm.
It is another further object of the present invention to provide a wafer centering device for centering a wafer processing machine that utilizes a wafer centering chuck equipped with locating pins and a wafer calibration disk equipped with an edge bead portion.
It is still another object of the present invention to provide a wafer centering chuck for centering a wafer processing machine for replacing a conventional vacuum chuck during a calibration procedure such that a visible reference can be provided.
It is yet another object of the present invention to provide a wafer centering chuck for centering a wafer processing machine that is equipped with a center locating pin for calibrating a liquid dispensing nozzle used in processing stations where a photoresist liquid or a developing liquid is dispensed.
It is still another further object of the present invention to provide a wafer calibration disk which is used to calibrate an edge bead rinse nozzle by a built-in edge bead portion provided on the calibration disk.
It is yet another further object of the present invention to provide a wafer calibration disk which can be used in conjunction with a wafer centering chuck for centering a wafer processing machine wherein the calibration disk is formed of a transparent material and equipped with a measuring grid at its center for providing a visible reference.